Summary

This study found that robotic-assisted functional alignment (FA) in total knee arthroplasty (TKA) leads to better intraoperative soft-tissue balance and larger tibial internal rotational angles relative to femur compared to navigation-assisted mechanical alignment (MA) TKA, suggesting that FA offers advantages in terms of both soft-tissue balancing and rotational kinematics during surgery.

Abstract

Introduction

Intraoperative soft-tissue balance can impact the rotational kinematics between the femur and tibia during total knee arthroplasty (TKA). Functional alignment (FA) is a newer surgical approach that aims to align the prosthesis based on each patient's bony anatomy, while adjusting soft-tissue balance using robotic assistance. This study aimed to evaluate intraoperative soft-tissue balance in robotic-assisted FA TKA compared to conventional navigation-assisted mechanical alignment (MA) TKA. The study also explored how soft tissue balance affects intraoperative rotational kinematics. The hypothesis was that FA would lead to better intraoperative soft-tissue balance and rotational kinematics than MA.

Method

This retrospective study involved 249 patients, with 111 undergoing robotic-assisted FA TKA and 138 undergoing navigation-assisted MA TKA. Both groups used the same implant with anatomical design. Soft-tissue balance was assessed by measuring the component gap (CG) both medially and laterally in flexion and extension using a femoral trial implant and implant-specific force-controlled tensioner with an equal distraction force. Extension or flexion balance was defined as the difference between medial and lateral CG in extension or flexion respectively and medial balance was defined as the difference medial CG between extension and flexion. A joint was considered well-balanced if the extension, flexion or medial balance was within 2 mm. Tibial internal rotational angles relative to the femur were recorded at full extension and 10-, 30-, 45-, 60-, 90-, and 120-degree knee flexion using robotics or navigation after implantation.

Results

Medial and lateral CG in extension was 10.1 ± 2.0 and 12.1 ± 2.7 in FA, and 10.1 ± 1.7 and 12.7 ± 2.3 in MA. Medial and lateral CG in flexion was 11.1 ± 2.5 and 13.1 ± 3.0 in FA, and 11.3 ± 1.7 and 13.2 ± 2.5 in MA. FA led to better extension balance compared to MA (2.0 mm vs. 2.6 mm, p=0.02). The proportion of well-balanced joints was higher in FA for both extension (65.5% vs. 49.3% in MA) and flexion (60.0% vs. 43.5% in MA). However, the proportion of medially-well-balanced joints was similar between FA and MA (78.3% vs. 80.9%). Additionally, Tibial internal rotational angles against femur at full extension and 10-, 30-, 45-, 60-, 90-, 120-degree flexion were 0.4 ± 6.6, 3.0 ± 7.0, 2.7 ± 7.4, 3.2 ± 7.2, 4.3 ± 7.1, 6.7 ± 7.1, 9.3 ± 6.8 in FA, and 0.9 ± 6.4, 3.2 ± 6.6, 2.0 ± 7.0, 1.8 ± 6.8, 2.2 ± 6.6, 3.8 ± 6.5, 7.2 ± 6.9 in MA, respectively. FA TKA compared to MA TKA exhibited significantly larger tibial internal rotational angles against the femur from 30- to 120-degree knee flexion (all, p<0.01). Larger tibial internal rotation at 45 and 60 degrees of flexion was also associated with well-balanced knees in flexion.

Conclusion

Robotic-assisted FA TKA achieved better intraoperative soft-tissue balance compared to navigation-assisted MA TKA. Moreover, larger tibial internal rotation was observed in FA TKAs and well-balanced knees in flexion. These findings suggest that FA may offer advantages over MA in terms of soft-tissue balancing and rotational kinematics during TKA.